Update ruby-mime-types to 1.21.

== 1.21 / 2013-02-09

* New MIME Types:
  * Various new or updated MIME types by Garret Alfert:
    application/vnd.ms-fontobject, .eot; application/x-chrome-extension, .crx;
    application/x-web-app-manifest\+json, .webapp; application/x-xpinstall,
    .xpi; image/svg\+xml, .svg, .svgz; image/webp, .webp; text/cache-manifest,
    .appcache, .manifest. https://github.com/halostatue/mime-types/pull/24
* Fixed some Manifest.txt related madness on Travis.

== 1.20.1 / 2013-01-26

* New MIME Types:
  * Apple iWork document types added by Hans de Graaff
    (application/x-iwork-keynote-sffkey, .key;
    application/x-iwork-pages-sffpages, .pages;
    application/x-iwork-numbers-sffnumbers, .numbers).
    https://github.com/halostatue/mime-types/issue/20
  * epub, ibooks, mobi, and DMG content types by Mauricio
    Linhares (mac:application/x-apple-diskimage, .dmg; application/epub\+zip,
    .epub; application/x-ibooks\+zip, .ibooks; application/x-mobipocket-ebook,
    .mobi). https://github.com/halostatue/mime-types/issue/22
  * rss content type by Garret Alfert (application/rss\+xml, .rss).
    https://github.com/halostatue/mime-types/issue/23
  * Added or updated MIME types from the latest IANA list.
* Fixed MIME Types:
  * Excel macro-enabled spreadsheets had an incorrect extension. Thanks to
    Rafael Belvederese for reporting this issue.
    https://github.com/halostatue/mime-types/issue/21
* Enabled for use with travis.
* Enabled gem signing.
* Fixed an error related to MIME type downloads.
* This was previously published as 1.20, but I had forgotten some attributions.

(taca)